1. Field of the Invention
The present invention relates to permeability offset of shield layers for correcting the bias of a free layer structure in a spin valve sensor and, more particularly, to such shield layers which exert a net image current field HIM for counterbalancing other fields acting on the free layer structure.
2. Description of the Related Art
The heart of a computer is an assembly that is referred to as a magnetic disk drive. The magnetic disk drive includes a rotating magnetic disk, a slider that has read and write heads, a suspension arm above the rotating disk and an actuator that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic impressions to and reading magnetic signal fields from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
An exemplary high performance read head employs a spin valve sensor for sensing the magnetic signal fields from the rotating magnetic disk. The sensor includes a nonmagnetic electrically conductive spacer layer sandwiched between a ferromagnetic pinning layer and a ferromagnetic free layer. An antiferromagnetic pinning layer interfaces the pinned layer for pinning the magnetic moment of the pinned layer 90xc2x0 to an air bearing surface (ABS) wherein the ABS is an exposed surface of the sensor that faces the rotating disk. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. A magnetic moment of the free layer is free to rotate upwardly and downwardly with respect to the ABS from a quiescent or zero bias point position in response to positive and negative magnetic signal fields from the rotating magnetic disk. The quiescent position of the magnetic moment of the free layer, which is preferably parallel to the ABS, is when the sense current is conducted through the sensor without magnetic field signals from the rotating magnetic disk. If the quiescent position of the magnetic moment is not parallel to the ABS the positive and negative responses of the free layer will not be equal which results in read signal asymmetry which is discussed in more detail hereinbelow.
The thickness of the spacer layer is chosen so that shunting of the sense current and a magnetic coupling between the free and pinned layers are minimized. This thickness is typically less than the mean free path of electrons conducted through the sensor. With this arrangement, a portion of the conduction electrons is scattered by the interfaces of the spacer layer with the pinned and free layers. When the magnetic moments of the pinned and free layers are parallel with respect to one another scattering is minimal and when their magnetic moments are antiparallel scattering is maximized. An increase in scattering of conduction electrons increases the resistance of the spin valve sensor and a decrease in scattering of the conduction electrons decreases the resistance of the spin valve sensor. Changes in resistance of the spin valve sensor is a function of cos xcex8, where xcex8 is the angle between the magnetic moments of the pinned and free layers. When a sense current is conducted through the spin valve sensor, resistance changes cause potential changes that are detected and processed as playback signals from the rotating magnetic disk. The sensitivity of the spin valve sensor is quantified as magnetoresistance or magnetoresistive coefficient dr/R where dr is the change in resistance of the spin valve sensor from minimum resistance (magnetic moments of free and pinned layers parallel) to maximum resistance (magnetic moments of the free and pinned layers antiparallel) and R is the resistance of the spin valve sensor at minimum resistance. Because of the high magnetoresistance of a spin valve sensor it is sometimes referred to as a giant magnetoresistive (GMR) sensor.
An improved spin valve sensor, which is referred to hereinafter as antiparallel pinned (AP) spin valve sensor, is described in commonly assigned U.S. Pat. No. 5,465,185 to Heim and Parkin which is incorporated by reference herein. The AP spin valve differs from the spin valve described above in that the pinned layer comprises multiple thin films, hereinafter referred to as AP pinned layer. The AP pinned layer has a nonmagnetic spacer film which is sandwiched between first and second ferromagnetic thin films. The first thin film, which may comprise several thin films, is immediately adjacent to the antiferromagnetic layer and is exchange-coupled thereto, with its magnetic moment directed in a first direction. The second thin film is immediately adjacent to the free layer and is exchange-coupled to the first thin film by the minimal thickness (in the order of 6 xc3x85) of the spacer film between the first and second thin films. The magnetic moment of the second thin film is oriented in a second direction that is antiparallel to the direction of the magnetic moment of the first film. The magnetic moments of the first and second films subtractively combine to provide a net moment of the AP pinned layer. The direction of the net moment is determined by the thicker of the first and second thin films. The thicknesses of the first and second thin films are chosen so that the net moment is small. A small net moment equates to a small demagnetizing (demag) field exerted on the free layer by the AP pinned layer. Since the antiferromagnetic exchange coupling is inversely proportional to the net moment, this results in a large exchange coupling between the pinning and pinned layers.
The transfer curve for a spin valve sensor is defined by the aforementioned cos xcex8 where xcex8 is the angle between the directions of the magnetic moments of the free and pinned layers. In a spin valve sensor subjected to positive and negative magnetic signal fields from a moving magnetic disk, which are typically chosen to be equal in magnitude, it is desirable that positive and negative changes in the resistance of the spin valve read head above and below a bias point on the transfer curve of the sensor be equal so that the positive and negative readback signals are equal. When the direction of the magnetic moment of the free layer is substantially parallel to the ABS and the direction of the magnetic moment of the pinned layer is perpendicular to the ABS in a quiescent state (no signal from the magnetic disk) the positive and negative readback signals should be equal when sensing positive and negative fields that are equal from the magnetic disk. Accordingly, the bias point should be located midway between the top and bottom of the transfer curve. When the bias point is located below the midway point the spin valve sensor is negatively biased and has positive asymmetry and when the bias point is above the midway point the spin valve sensor is positively biased and has negative asymmetry. The designer strives to improve asymmetry of the readback signals as much as practical with the goal being symmetry. When the readback signals are asymmetrical, signal output and dynamic range of the sensor are reduced.
Readback asymmetry is defined as             V      1        -          V      2            max    ⁡          (                        V          1                ⁢                  xe2x80x83                ⁢        or        ⁢                  xe2x80x83                ⁢                  V          2                    )      
For example, +10% readback asymmetry means that the positive readback signal V1 is 10% greater than it should be to obtain readback symmetry. 10% readback asymmetry is acceptable in many applications. +10% readback asymmetry may not be acceptable in applications where the applied field magnetizes the free layer close to saturation. In these applications +10% readback asymmetry can saturate the free layer in the positive direction and will reduce the negative readback signal by 10%. An even more subtle problem is that readback asymmetry impacts the magnetic stability of the free layer. Magnetic instability of the free layer means that the applied signal has disturbed the arrangement or multiplied one or more magnetic domains of the free layer. This instability changes the magnetic properties of the free layer which, in turn, changes the readback signal. The magnetic instability of the free layer can be expressed as a percentage increase or decrease in instability of the free layer depending upon the percentage of the increase or decrease of the asymmetry of the readback signal. Standard deviation of the magnetic instability can be calculated from magnetic instability variations corresponding to multiple tests of the free layer at a given readback asymmetry. There is approximately a 0.2% decrease in standard deviation of the magnetic instability of the free layer for a 1% decrease in readback asymmetry. This relationship is substantially linear which will result in a 2.0% reduction in the standard deviation when the readback asymmetry is reduced from +10% to zero. The magnetic instability of the free layer is greater when the readback asymmetry is positive than when the readback asymmetry is negative.
When the sense current IS is applied to the spin valve sensor there is an image sense current in each of the first and second shield layers. The image sense current in each shield layer causes each shield layer to produce an image sense current field HIM which traverses the free layer in a direction that is substantially perpendicular to the ABS. When the free layer of the AP pinned spin valve is symmetrically located midway between the first and second shield layers the image sense current fields counterbalance each other so that the net image sense current field on the free layer is zero. When the free layer is located asymmetrically between the first and second shield layers, hereinafter referred to as gap offset, a net image sense current field can be employed for counterbalancing the other magnetic fields on the free layer. This is accomplished by sizing the first and second gap layers that separate the free layer from the first and second shield layers respectively so that the free layer is closer to a selected one of the shield layers. It is preferred that the second gap be thinner than the first gap so that the free layer is closer to the second shield layer. When these thicknesses are carefully selected readback asymmetry can be improved so that magnetic stability of the free layer is optimized.
With increasing linear densities of magnetic read heads, a gap offset becomes impractical because of the risk of shorting between first and second lead layers to the spin valve sensor and the shield layers. For instance, in a bottom spin valve, where the free layer structure is closer to the second shield layer than to the first shield layer, the second read gap is typically narrower than the first read gap so that the second shield layer exerts a net imaging current field HIM on the free layer structure for counterbalancing other fields acting thereon. If this second read gap gets too narrow the thickness of the first read gap layer (G1), which is composed of alumina, will be too thin to prevent the lead layers from shorting to the second shield layer. Since the total first read gap is made narrower in order to promote higher linear density of the read head, it becomes difficult to make a gap offset without shorting the lead layers to the second shield layer. The opposite situation is true for a top spin valve where the free layer structure is closer to the first shield layer than to the second shield layer.
In a dual spin valve sensor where the free layer structure is centered between first and second pinned layer structures the demagnetizing field HD acting on the free layer structure is less than that in a single spin valve sensor. Accordingly, the imaging field HIM in a dual spin valve sensor is important for counterbalancing ferromagnetic coupling fields HFC exerted by the pinned layer structures on the free layer structure.
It is further desirable to employ a metallic pinning layer with the preference being platinum manganese (PtMn). A metallic pinning layer in a single spin valve sensor causes an additional sense current field on the free layer structure and top and bottom metallic pinning layers in a dual spin valve sensor cause the sense current fields therefrom to be substantially counterbalanced. As indicated hereinabove, the preferred pinned layer structure for either the single or dual spin valve sensor is an AP pinned layer structure. It is desirable in any of these embodiments that the second AP pinned layer, which interfaces the spacer layer, be the thicker of the first and second AP pinned layers of the AP pinned layer structure for increasing the magnetoresistive coefficient dr/R of the spin valve sensor. Since a platinum manganese pinning layer causes a negative ferromagnetic coupling field from the second AP pinned layer on the free layer structure this field will be additive to the net demagnetizing field HD from the second AP pinned layer. Accordingly, this increases the biasing of the free layer structure in one direction which needs to be counterbalanced in order to obtain read signal symmetry. Accordingly, in order to obtain maximum magnetoresistive coefficient dr/R with one or more platinum manganese (PtMn) pinning layers it important that there be a net imaging current field HIM for counterbalancing other fields acting on the free layer structure.
In summary, the location of the transfer curve relative to the bias point is influenced by four major forces on the free layer of a spin valve sensor, namely a ferromagnetic coupling field HFC between the pinned layer and the free layer, a net demag field HD from the pinned layer, a sense current field HI from all conductive layers of the spin valve except the free layer and a net image current field HIM from the first and second shield layers. There is a need to deal with these forces on the free layer so as to improve asymmetry of the readback signals.
The present invention provides a net imaging current field HIM from the first and second shield layers without the necessity of making a gap offset for the purpose of properly biasing the free layer. This is accomplished by making the permeability of one of the first and second shield layers greater than the permeability of the other of the first and second shield layers. Accordingly, the first and second shield layers are made of two different materials in order to provide this permeability offset. It should be understood that permeability xcexc=4 xcfx80MS÷HK where MS is the saturation magnetization of the material and HK is the uniaxial anisotropy. Uniaxial anisotropy is the amount of applied field required to rotate a magnetic moment of the material 90xc2x0 from its easy axis. Permeability is a measure of how easy it is to magnetize the material. The higher the permeability, the softer the material. Accordingly, the softer of the two materials is placed in the shield layer where it is desired to obtain the greatest image current field HIM. It can be seen from the above formula that the uniaxial anisotropy HK is inversely proportional to the permeability xcexc. The preferred high permeability materials are nickel iron (NiFe) based excluding cobalt (Co) and the low permeability materials are cobalt based. As an example the uniaxial anisotropy HK of nickel iron (NiFe) can be between 1 to 5 Oe and the uniaxial anisotropy HK of nickel iron cobalt (NiFeCo) can be between 5 to 30 Oe. The uniaxial anisotropy HK of nickel iron cobalt (NiFeCo) is roughly proportional to the atomic percent of the cobalt (Co). For instance, if the cobalt is 10% in NiFeCo then the HK is about 10 Oe.
In a bottom spin valve sensor the permeability of the second shield layer is higher than the first shield layer since the free layer structure is typically closer to the second shield layer and is subjected to a net sense current field from conductive layers therebelow. In a top spin valve sensor where the free layer structure is typically closer to the first shield layer the permeability of the first shield layer is higher than the second shield layer since the net sense current field is due to the conductive layers above the free layer structure. In a dual spin valve sensor the net sense current field is low because the conductive layers above and below the free layer structure are essentially the same. However, a net ferromagnetic coupling field and a net demagnetizing field may have to be dealt with which requires a net image current field in order to obtain proper biasing of the free layer structure. These factors are driven by the desire to locate the thicker of the first and second AP pinned layers next to the spacer layer and to employ a pinning layer composed of platinum manganese (PtMn) which has a high blocking temperature.
An object of the present invention is to provide a read head with a net imaging current offset HIM without increasing a gap offset in order to properly bias the free layer.
Another object is to provide an imaging current offset HIM that promotes linear density in a spin valve sensor that employs a platinum manganese (PtMn) pinning layer in an AP pinned layer structure wherein the second AP pinned layer is thicker than the first AP pinned layer and interfaces the spacer layer.
Other objects and advantages of the invention will become apparent upon reading the following description taken together with the accompanying drawings.